To have rib already harvested from both sides, and then need more, is really unusual. Have you had second and third opinions about what to do, what's needed, what the chances are of significant improvement, what the alternatives are? If you are happy that your doctor is a true expert and he recommends another rib, then if you trust him to perform what sounds like an extremely complicated operation, you should trust his advice on how to get the cartilage that he needs. Acquiring the cartilage is probably much simpler than whatever is planned for your nose. If you're very concerned, ask him about the pros and cons and alternatives (such as banked cartilage) in his hands; he'll probably have good answers.
